Rigid low cost solar thermal panels

ABSTRACT

A rigid solar thermal panel created from thin flexible materials, including a solar absorber layer of a metal sheet coated with a solar absorbent material having dimples, a tensioned optical film above an upper surface of the solar absorber layer, and an insulation layer. The insulation layer is spaced apart from a lower surface of the solar absorber layer to form a cavity. The dimples project into the cavity and at least one of the dimples is in contact with the insulation layer, thereby providing rigid support for the rigid solar thermal panel.

BACKGROUND

Solar thermal panels capture solar radiation in the form of heat. An absorber, typically a metal plate or foil coated with a specialized solar absorber or black paint, converts incident solar radiation into heat through the process of solar absorption. The absorbed heat can be used in many ways, including to directly heat air in a forced air heating system or water in a forced hot water system, or to evaporate a fluid in an absorption chiller of a refrigeration cycle. The amount of solar energy available for use is described by what is known as the solar constant, which is a measure of unit energy per unit time per unit area of incoming solar electromagnetic radiation. The solar constant is typically represented in kilowatts per square meter (kW/m²), and the average value of the solar constant has been determined to be about 1.361 kW/m². Accordingly, over the span of an hour, a one square meter solar panel with 100% conversion efficiency would generate approximately 1.361 kWh, which is equivalent to about 4435 British Thermal Units (BTUs).

All solar thermal panels have less than 100% collector efficiency due to energy and power loss and dissipation. Solar thermal panels that are regarded as efficient (e.g., having peak efficiencies of at least about 80%) typically have a rigid design fabricated from an aluminum or copper metal layer coated with a solar absorbent material, a rigid metal frame, a rigid insulator, and rigid glazing comprised of an expensive low-iron solar glass. Because a solar thermal panel typically requires air gaps between the glazing and absorber layers and the absorber and insulation layers, rigid solar glass and rigid metal support structures are often used to provide and maintain such air gaps. As a result, a large solar thermal panel, particularly with glass glazing, would weigh too much for economical fabrication, shipping, and installation. Because of weight and size limitations of the rigid glass and frame materials, the largest practical solar thermal panels have generally been less than 32 square feet, and panels of that size have weighed several hundred pounds. Even with a flexible glazing (e.g., optical film) in place of solar glass, the rigid frame of a typical solar thermal panel is heavy and limited in length, restricting the panel length to just a few feet. There exists a need for high-efficiency solar thermal panels that are more light-weight and cost-efficient.

SUMMARY OF THE INVENTION

The present invention provides for solar thermal panels made from thin flexible materials that can be lighter weight, rigid, and highly efficient. Methods of manufacturing rigid solar thermal panels from flexible materials are also provided.

A rigid solar thermal panel comprises a solar absorber layer comprising a metal sheet coated with a solar absorbent material, a tensioned optical film above an upper surface of the solar absorber layer, and an insulation layer. The insulation layer is located below and spaced apart from a lower surface of the solar absorber layer to form a cavity in the rigid solar thermal panel. The metal sheet includes dimples that project into the cavity. At least one of the dimples is in contact with the insulation layer, providing rigid support for the rigid solar thermal panel.

A method of manufacturing a rigid solar thermal panel includes forming dimples in a metal absorber sheet and forming side walls along at least two edges of the metal absorber sheet to form a solar absorber layer and placing an optical film on a surface of the solar absorber layer. The method further includes applying a force substantially perpendicular to the surface of the solar absorber layer to cause the metal absorber layer to bow and to secure the optical film on either side of the solar absorber layer proximate to the side walls. The bowing can facilitate the mechanical joining of solar thermal panel components, including the optical film to the solar absorber layer. The force is released to permit the solar absorber layer to return to its pre-bowed state and to tension the optical film across the surface of the metal absorber layer.

Embodiments of the present invention have many advantages. Rigid solar panels as described herein provide high-efficiency performance panels with lower manufacturing, transportation, and installation costs as a result of lighter component materials, which can result in lower weight panels.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

FIG. 1 is a perspective view of a rigid solar panel mounted on an exterior wall of a structure.

FIG. 2 is an exploded view of the rigid solar panel of FIG. 1.

FIG. 3 is a cutaway view of an absorber panel illustrating dimples and a cavity of the panel.

FIG. 4 is a detailed view of a flange and spring form at an edge of a solar thermal panel.

FIG. 5 is an exploded view of a rigid solar thermal panel mounted on a support base.

FIG. 6 is a perspective view of a support base and a support frame supporting a rigid solar thermal panel.

FIG. 7 is a detailed view of the support frame of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

A description of example embodiments of the invention follows.

FIG. 1 shows rigid solar thermal panel 1 mounted on a flat support structure, such as an exterior wall of a building or wood panel 2. The rigid solar thermal panel 1 includes a solar absorber layer 8 (FIG. 4) having dimples 13, 14. The solar absorber layer 8 can be formed from a metal, such as a sheet of aluminum, coated with a solar absorbent material, such as silicon dioxide. Covering the solar absorber layer 8 is a flexible optical film 4 (FIG. 2). The optical film 4 is mechanically attached to flanges 5 and tensioned by spring forms 6 located at each side 3 of the solar thermal panel 1. FIG. 4 is a more detailed illustration of the construction of a flange 5 and spring form 6. The spring forms 6 mechanically tension the flexible optical film 4 across the upper surface of the solar absorber layer 8 at sides 3. The remaining sides of the panel 1 may be sealed with rigid gaskets 7. The gaskets 7 can create a sealed and airtight rigid solar panel assembly 1. Gaskets 7 can be formed from, for example, a high durometer elastomer material or extruded polymer or plastic material. Although the rigid solar thermal panel 1 is illustrated in FIG. 1 as having a rectangular shape with sides 3, flanges 5, and spring forms 6 located along the longer edges of the panel 1 and with gaskets 7 located along shorter edges of panel 1, different configurations are possible. For example, sides 3, flanges 5, and spring forms 6 could be located along the shorter edges of the panel 1, and gaskets 7 could be located along the longer edges of the panel 1. Alternatively, the rigid solar thermal panel 1 could have varied shapes, including, for example, square, triangular, other polygonal shapes, shapes that include curves, or irregular shapes, with tensioning mechanisms (e.g. flanges 5 and spring forms 6) located at some edges and gaskets (e.g. gaskets 7) at other edges.

FIG. 2 illustrates an exploded view of rigid solar panel 1. Before the optical film 4 is attached to the solar absorber layer 8 at flanges 5, two wear strips 9 and 10 (e.g., thin metal or plastic strips) may be mechanically or chemically bonded to the top and bottom of the optical film 4. The optical film 4 creates a flexible, lightweight glazing for the rigid solar panel 1. The wear strips 9 bonded to the top of the optical film 4 prevent frictional wear on the film 4 that can be caused from tensioning the spring forms 6. The wear strips 10 bonded to the bottom of the optical film 4 protect against frictional wear that can be caused from contact with the surface of the metal solar absorber layer 8. Most of the frictional wear to the optical film 4 can occur at the edge of the solar absorber layer 8 at positions 11, where there is a bend between an upper surface of the solar absorber layer 8 and walls 3. To alleviate frictional wear to optical film 4, solar absorber layer 8 can have a curved edge at position 11, as shown in more detail in FIG. 4. Beneath the solar absorber layer 8 is an insulation layer 12 that prevents heat loss to the mounting surface of wall 2. The insulation layer 12 can also serve as the bottom surface of the air channel through the solar thermal panel 1.

FIG. 3 is a cutaway view of solar thermal panel 1 mounted on a support surface 2. Dimples 13 and 14 of the solar absorber layer 8 extend from the upper surface of the solar absorber layer toward the insulation layer 12 and provide air gaps 15 between the bottom surface of the transparent optical film 4 and the top surface of absorber layer 8. As illustrated in FIG. 3, dimples 14 have a depth that is approximately equal to the height of wall 3 of the solar absorber layer 8, so that they are in direct contact at their base with insulation layer 12. The contacting dimples 14 provide rigid support for the solar thermal panel 1 and create an air baffle structure beneath the bottom surface of the absorber layer 8. The cavity 16, which is the space between the solar absorber layer 8 and the insulation layer 12, is structurally maintained, at least in part, by the dimples 14 and serves as an air channel through which air that is heated by solar radiation travels. Additionally, the air baffle structure created by dimples 14 causes air that is being heated to travel a longer and more circuitous route through the air channel from an inlet duct to an outlet duct (not shown) of the solar thermal panel 1, thereby increasing an amount of time that the air is in contact with the solar absorber layer 8.

Solar absorber layer 8 can include dimples of varying dimensions. As shown in FIG. 3, non-contacting dimples 13, located among the contacting dimples 14, can extend to a depth that does not reach the insulation layer 12. Dimples 13 and 14 can have a wide array of cross-sectional diameters. Dimples 13 and dimples 14 can be interspersed to create additional air baffle structure in cavity 16. Non-contacting dimples 13 can project into cavity 16 a shorter distance than contacting dimples 14. For example, non-contacting dimples 13 can project a distance that is less than about 75% of the height of wall 3, less than about 50% of the height of wall 3, or less than 25% of the height of wall 3. Non-contacting dimples can have a truncated spherical dimension based on available surface area between larger dimples. Dimples 13, 14, in addition to creating air baffles, also increase the available heat transfer surface area of the solar absorber layer 8. For example, including dimples can increase the available surface area for air contact by 10% to about 30%, or of about 20% to 25% as compared with a flat solar absorber layer. As illustrated in FIGS. 1, 3, the inclusion of dimples 13, 14 increases the available surface area for air contact by about 22% over a flat solar absorber layer. The solar absorber layer can have a dimpled area of at least about 80% of a total surface area of the solar absorber layer, or of about 80% to about 95% of the total surface area (e.g., 78%, 80%, 85%, 87%, 90%, 95%, 97%). For example, about 50% to about 70% of the dimpled area can include contacting dimples (e.g., 48%, 50%, 55%, 65%, 70%, 72%) and about 20% to about 40% of the dimpled area can include shorter dimples (e.g., 18%, 20%, 22%, 25%, 30%, 35%, 40%, 42%). Non contacting dimples may be optional.

While FIGS. 1, 3, and 4 illustrate solar absorber layer 8 with dimples 13, 14 having substantially hemispherical shapes, other shapes could be included in place of, or in addition to, the rounded dimples illustrated. For example, dimples can have cross-sectional shapes that are triangular, square, rectangular, or irregularly shaped. Dimples can project from the surface to a greater and lesser extent such that they form shallower or deeper indentations in solar absorber layer 8. Also, as illustrated in FIG. 1, dimples 13 and 14 can be arranged in rows. Other patterns and dimple sizes are permissible. For example, a solar thermal panel could include a single row of larger dimples 14 and several adjacent rows of smaller dimples 13. Alternatively, dimples 13, 14 could be arranged in a random pattern. Additionally, a solar absorber layer 8 can include varying sizes of contacting dimples (e.g. dimples 14) and noncontacting dimples (e.g., dimples 13). For example, a pattern can include two, three, four, or more differently dimensioned dimples that contact insulation layer 12. Similarly, a pattern can include two, three, four, or more differently dimensioned smaller dimples that are interspersed between or around the contacting dimples.

The number of dimples included in a solar thermal panel can be dependent on the desired overall size of the thermal panel. For example, a panel of 2.2 m² can include about 27 larger dimples each having a diameter of about 0.21 m and about 32 smaller dimples each having a diameter of about 0.1 m. In addition, or alternatively, the solar absorber layer can include a dimpled area of, for example, at least about 70%, at least about 80% or at least about 90% of a total exposed area of the solar absorber layer. Smaller dimples can be interspersed among the larger dimples, such that the solar absorber layer includes an adequate number of larger dimples to provide structural support to the solar thermal panel while maximizing use of the area surrounding the larger dimples by including smaller dimples that provide additional air baffling.

Solar absorber layer 8 can be formed from a sheet of metal, such as aluminum, copper or other metal, that is coated with a solar absorber, such as silicon dioxide or titanium dioxide. The silicon dioxide coating can be formed from, for example, quartzite gravel or crushed quartz that has been purified. Dimples can be formed in a flexible material, such as sheets of aluminum or copper, with known metal forming techniques.

Optical film 4 can be a flexible, transparent film that permits a high percentage (e.g., at least about 80%, at least about 85%, at least about 90%, or at least about 95%) of incident solar radiation to pass through the film. For example, optical film 4 can be composed of a thin fluoropolymer material, such as a sheet of polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), which is capable of withstanding high temperatures and is optically transparent to about 90% to about 95% of incident solar radiation. Examples of such optical films include Tefzel® (Dow Chemical, Marlborough, Mass.), Kynar® (Arkema, King of Prussia, Pa.) and Norton® (Saint-Gobain, Worcester, Mass.). Films formed from PTFE or PVDF are also durable, flexible, tear-resistant, and lightweight. An optical film 4 formed from PTFE or PVDF film can have a thickness of about 0.5 mil (0.013 mm) to about 50 mil (1.27 mm), or of about 1 mil (0.025 mm).

Heat transfer in a solar thermal panel is a function of radiation, conduction, and convection. Radiation exchange, or the transfer of heat through a vacuum or gas, is the primary method used to capture solar radiation and convert it to heat in a solar thermal panel. Solar radiation can pass directly through any optically transparent material, such as glass or PTFE or PVDF film. An ideal solar absorber would have 100% absorptivity and 0% emissivity, indicating that 100% of solar radiation is captured and converted to heat. However, typical high-performing solar absorbers have specifications that indicate ranges of about 85% to about 95% absorptivity and about 5% to about 10% emissivity. Solar thermal panels of the present invention, which include optical films of PTFE or PVDF as a glazing layer, are configured to enable absorption of about 90% to about 95% of incident solar radiation.

Once the radiation is converted into heat at the surface of the solar absorber layer (e.g. solar absorber layer 8), the heat can be lost via conduction or convection. The glazing layer located above the solar absorber surface minimizes convection heat losses. Conduction heat losses occur when the glazing and solar absorber surfaces are in direct physical contact. To eliminate or reduce conduction losses, a solar thermal panel can include an air gap between the absorber surface and the glazing. The dimples (e.g., dimples 13, 14) in solar thermal panels advantageously create air gaps between the glazing layer (e.g. optical film 4) and the surface of the solar absorber layer (e.g., solar absorber layer 8), thereby reducing conduction heat losses while simultaneously providing air baffles and structural support to the solar absorber thermal panel.

Following absorption, heat transfers from the surface of the solar absorber to a space to be heated (e.g., cavity 16 located between a bottom surface of solar absorber layer 8 and an upper surface of insulation layer 12) via convection with a transfer fluid of either liquid (e.g., water) or gas (e.g., air). The larger the heat transfer surface area in a solar thermal panel that the transfer fluid contacts, the more efficient the heat transfer process becomes. In an air-based solar thermal panel, the air typically transverses the length of the bottom side of the solar absorber layer. Air enters at the bottom of the panel through an inlet duct and out of the top of the panel through an outlet duct (e.g. air ducts such as air connection 19 as shown in FIG. 5). In embodiments of the present invention, the series of air baffles, formed, for example, by dimples 13, 14, direct the flow of air or liquid across the surface area of the bottom side of the solar absorber layer 8 between inlet and outlet ducts of the thermal panel 1. By creating turbulence and providing additional surface area, the dimples 13, 14 lengthen the path of airflow through the panel and thereby increase panel efficiency. Solar thermal panels of the present invention can have conversion efficiencies ranging from about 75% to about 95% (e.g., 74%, 75%, 85%, 80%, 85%, 90%, 92%, 95%, 96%).

Solar thermal panels can be formed from flexible, lightweight materials, minimizing the use of heavy materials, such as heavy structural frames and solar glass, and can be less expensive to manufacture and install. Structural rigidity in solar thermal panels of the present invention is provided, at least in part, by dimples that extend from an upper surface of a solar absorber layer to an upper surface of an insulation layer, thereby preventing deformation of the panel and, further, supporting the glazing layer of the panel.

Prior attempts at reducing the cost of solar thermal panels have included replacing low-iron solar glass with rigid sheets of transparent, polycarbonate plastic, such as Lexan™ (Sabic, Pittsfield, Mass.), or ultra-thin, high-temperature optical films, including polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) optical films. For example, as described in U.S. Pat. No. 9,243,815 to Sylvan, the entire contents of which are incorporated herein by reference, the rigid glass glazing of a solar panel is replaced with a lightweight optically transparent film. Additionally, as also described in U.S. Pat. No. 9,243,815 to Sylvan, materials, such as metal foils and thin ceramic oxide paper insulator layers have been used to create flexible solar thermal panels. However, for some applications it can be advantageous to use a rigid panel.

Solar thermal panels of the present invention are rigid, yet lightweight. For example, a 24 ft² rigid solar thermal panel of the present invention can weigh about 7 lbs to about 12 lbs. A similarly sized conventional rigid solar thermal panel weighs about 90 lbs.

To support the glazing layer and prevent optical films from sagging or tearing, solar thermal panels of the present invention can include a tensioning mechanism to tension the optical film over the upper surface of the solar absorber layer. FIG. 4 is an end view of the solar thermal panel 1 with the end gasket 7 removed. When assembled, the optical film 4, the top wear strip 9, and bottom wear strip 10 are mechanically captured between the spring forms 6 and the flanges 5. Furthermore, spring forms 6 can apply vertical spring force to optical film 4. The vertical spring force tensions the optical film 4 across the face of the absorber layer 8. The spring forms 6 permit the film to expand or contract based on fluctuations in temperature and positive or negative air pressure. Most of the friction caused by panel expansion or contraction is concentrated at the rounded corner edge at position 11 of the solar absorber layer 8. As illustrated, the corner of the solar absorber layer 8 at position 11 has a curved edge, which can more evenly distribute force and decrease wear between the bottom surface of wear strip 10 and the top surface of solar absorber layer 8.

To provide extra rigidity to the vertical sides 3 of the solar absorber layer 8, L-shaped indentations 17 or embossed right angle indentations can be incorporated into the metal absorber layer 8 during the manufacturing process. This rigidity enhancing technique can be further understood with reference to metal-forming operations, such as in the manufacturing of angle irons and sheet metal enclosures. As applied to solar thermal panels, the right angle indentations 17 strengthen the vertical wall 3 of the metal absorber layer 8 and can assist with maintaining a precise right angle between flange 5 and the vertical wall 3, further maintain the structural integrity of the solar thermal panel 1.

With spring forms 6 and gaskets 7, the metal solar absorber layer 8 and optical film 4 are held tightly against insulation layer 12. Insulation layer 12 can be, for example, a ceramic or fiberglass mat. As it is unsafe to use fiberglass or ceramic fiber directly as an insulation layer (i.e., without a cover layer) in a forced air system due to the potential dispersal of ceramic or glass fibers into the airflow, insulation layer 12 can include metal foil bonded to the top surface, or to the top and bottom surfaces, of the mat. With a metalized material added to the surface(s) of the fiberglass or ceramic mat (e.g., aluminum foil or other thin metal material), the insulation layer 12 can function as the bottom layer of the solar thermal panel 1. Thus, a cover layer of metal, in addition to providing an additional layer for insulation, functions to prevent the dispersal of contaminants from the mat into the airflow.

FIG. 5 shows the solar thermal panel 1 mounted on a support base 18. A wide variety of support bases can be used with varying features. Support base 18 can be a semi-rigid material such as, for example, a thermoformed plastic. A semi-rigid material can be any material that is solid and inelastic, but that can be capable of withstanding a nominal amount of deformation without breaking. The support base 18 can directly incorporate air connections 19 for connecting air ducts to permit transit of air through panel 1. The light weight of solar thermal panel 1 permits the panel to be installed on a support base 18 with little or no structural reinforcement. A panel with glass glazing could be too heavy and too rigid to be supported by a similar design. The side walls 20 and channels 21 can provide structural rigidity to minimize flexing of the base 18 and, consequently, panel 1. Side walls 20 and channels 21 can be incorporated directly into the thermoforming process of support base 18. Insulation layer 12 can protect the thermoformed base 8 from heat deflection created by high interior temperatures of solar thermal panel 1.

FIG. 6 illustrates support base 18 mounted on a support stand 22. Support stand 22 includes structural features such as a vertical post 24, horizontal cross braces 25, center brace 23 with an integral hinge mechanism, and bottom mount plate 26. Support stand 22 can be fabricated from steel or other metal materials. The vertical post 24 may be bolted to a roof or a paved surface using a mount plate, such as mount plate 26. Alternatively, the vertical post 24 can be partly buried in the ground. A pivot hinge mechanism on center base 23 connects the support base 18 and solar thermal panel 1 to the vertical post 24. The connection to center base 23 with an integral hinge mechanism permits the support base 18 and solar thermal panel 1 to be supported and to pivot in order to accommodate variations in the angle of the sun over the course of a solar year. As illustrated in FIG. 6, there are two air connections 19, or duct attachment features, that permit direct connection to an air duct. One air connection can operate as an inlet duct and the other as an outlet duct. In this manner, multiple panels can be connected to one another in series or in parallel, for example, with airflow connections between an outlet of one panel and an inlet of another panel. The duct attachment features could be round, square, or other shape.

FIG. 7 is a detailed view of the support stand 22. A cylindrical component 27, such as a bolt or hinge pin, connects the center brace 23 to the vertical post 24. The bolt or hinge pin 27 provides one axis of motion for the support base 18 and solar thermal panel 1. Also, center brace 23 can include a series of holes 28 arranged on a radius from the center support bolt or hinge pin 27. A locking pin or bolt 29 prevents the center brace 23 from pivoting when it is installed. When the locking pin or bolt 29 is removed, center brace 23 is free to rotate. As illustrated in FIG. 7, the radial hole pattern 28 can include holes arranged at 10 degrees increments, permitting the solar thermal panel 1 and support base 18 to be easily oriented towards the sun in a measured manner. Other implementations of a rotational hinge are possible, including a ratchet or other gear mechanism. In place of a manual pivot system, an electromechanical or hydraulic system could alternatively be used.

To increase assembly speed and decrease manufacturing costs, solar thermal panels can be substantially formed from materials that are available as roll stock. Roll stock is typically a thin material cut to a specific width and rolled onto a hollow or solid core. In production, the materials are unwound from the core as continuous sheets and are then processed in that format.

In an embodiment of this invention, an automated manufacturing system feeds lightweight, absorber roll stock (e.g., metal sheets coated with solar absorbent material), spring roll stock (e.g., stainless steel sheets), wear strips (e.g. aluminum foil), and optical film (e.g., PTFE or PVDF film) into a metal former, metal sealer and slitter. The metal former shapes the flat absorber roll stock into a panel having three-dimensional indentations (e.g., metal solar absorber layer 8 with dimples 13, 14). The manufacturing system can also, optionally, form side walls (e.g. walls 3) and flanges (e.g., flanges 5) from the flat metal absorber roll stock. Alternatively, side walls and flanges can be created from separate materials that are welded or otherwise affixed to the solar absorber layer. The manufacturing system can also add indentations (e.g. indentations 17) to the vertical sides and flanges (e.g., sides 3 and flanges 5) of the solar absorber layer to increase rigidity and support a right angle configuration between the flanges and vertical sides of the solar absorber layer.

Because the tensioned optical film can experience significant wear to wind and temperature expansion and contraction, two wear strips (e.g., metal strips 9, 10) formed of, for example, aluminum, can be applied to the edges of the optical film, protecting the film from directly contacting either the aluminum solar absorber top surface or the tensioning spring bottom surface, or both. The use of wear strips is further shown and described in U.S. Pat. No. 9,243,815 to Sylvan, the entire contents of which are incorporated herein by reference.

The wear strips may be mechanically bonded to both sides of the optical film by ultrasonic welding techniques or metal clinching systems, or the wear strips may be chemically bonded by adhesives, thereby creating a unified optical film and metal assembly. The optical film with applied wear strips can then be interposed beneath a tensioning spring (e.g., spring forms 6) and a flange (e.g., flange 5) of the formed solar absorber panel (e.g., metal solar absorber layer 8). The optical film assembly can then be mechanically clamped to the flange by welding, riveting, and/or by other metal-joining techniques to generate a tight bond. The joining can form a substantially airtight seal.

During the manufacturing process, it may be desirable or necessary to bow the solar absorber layer and flanges to facilitate the mechanical joining of these elements. Advantageously, the flexible nature of the absorber material (e.g., low gauge metal of, for example, 0.3 or 0.4 mm) permits bowing the materials during manufacturing of the panel. Bowing to facilitate assembly of a panel can be performed by applying pressure substantially perpendicular to the panel surface. When in the bowed state, automated assembly equipment can mechanically bond the optical film with attached wear strips to the two horizontal flanges of the solar absorber panel. Upon release of the perpendicular force, the elements return to their pre-bowed state and the optical film is tensioned over the upper surface of the solar absorber layer. Because the solar absorber material is typically of a light metal gauge, the solar thermal panel may not be fully tensioned until the entire assembly is attached to a support surface (e.g., insulation layer 12 and/or support base 18) and the angle between the vertical sides of the panel are adjusted to be at approximately 90 degrees with the flanges.

Constructing a solar thermal panel from flexible materials permits the panel to be incorporated into a low cost, non-rigid base, such as, for example, a thermoformed composition or similar technology (e.g., support base 18). With rigid solar panels that incorporate glass as a glazing, the support frame must be rigid enough to prevent flexing of the solar panel. Any flexing of the panel would cause the glazing to crack. By making a rigid solar panel with film glazing and flexible materials, the solar thermal panel can be less expensive and simpler to deploy. In place of rigid and expensive mounting frame, the rigid solar thermal panel can be mounted on a low cost thermoformed support panel. This type of support panel can then be mounted on a wall or roof of a structure, or attached to a support stand for ground installation. With a stand-based support, the panel's angle to the sun can be adjusted over the solar year to optimize solar thermal performance. For example, in the winter, the panel can be oriented to a more vertical position, and in the summer, a more horizontal position.

The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. A rigid solar thermal panel comprising: a solar absorber layer comprising a metal sheet coated with a solar absorbent material; a tensioned optical film above an upper surface of the solar absorber layer; and an insulation layer below and spaced apart from a lower surface of the solar absorber layer to form a cavity, the metal sheet having dimples projecting into the cavity and at least one of the dimples being in contact with the insulation layer, the contact of the at least one dimple with the insulation layer providing rigid support for the rigid solar thermal panel.
 2. The rigid solar thermal panel of claim 1, wherein the dimple is substantially hemispherical.
 3. The rigid solar thermal panel of claim 1, wherein the optical film is a film comprising polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF).
 4. The rigid solar thermal panel of claim 1, wherein the metal sheet is aluminum.
 5. The rigid solar thermal panel of claim 1, wherein the metal sheet is copper.
 6. The rigid solar thermal panel of claim 1, wherein the solar absorbent material is silicon dioxide.
 7. The rigid solar thermal panel of claim 1, further comprising a spring tensioning system that tensions the tensioned optical film over the upper surface of the solar absorber layer.
 8. The rigid solar thermal panel of claim 7, further comprising wear strips captured between a spring form along at least one edge of the optical film and a flange of the solar absorber layer, the wear strips configured to protect the optical film from frictional wear.
 9. The rigid solar thermal panel of claim 1, wherein the dimples comprise dimples that contact the insulation layer and additional dimples that project into the cavity a shorter distance than the contacting dimples.
 10. The rigid solar thermal panel of claim 9 wherein the additional dimples are interspersed in the metal layer among the contacting dimples.
 11. The rigid solar thermal panel of claim 1, wherein the solar absorber layer has a dimpled area of about 80% to about 90% of a total surface area of the solar absorber layer.
 12. The rigid solar thermal panel of claim 11, wherein about 50% to about 70% of the dimpled area comprises contacting dimples and about 20% to about 40%% of the dimpled area comprises shorter dimples.
 13. The rigid solar thermal panel of claim 9, wherein the shorter dimples project a distance that is less than about 75% of the height of the cavity of the solar thermal panel.
 14. The rigid solar thermal panel of claim 1, wherein the solar absorber layer further comprises integrated side walls and a curved bend between an upper surface of the solar absorber layer and the integrated side walls, the curved bend configured to minimize wear to the optical film.
 15. The rigid solar thermal panel of claim 14, wherein the solar absorber layer further comprises integrated flanges at each side wall.
 16. The rigid solar thermal panel of claim 15, wherein each of the integrated side walls and flanges includes at least one L-shaped indentation configured to maintain a right angle between the side wall and the flange.
 17. The rigid solar thermal panel of claim 1, further comprising a support base beneath the insulation layer.
 18. The rigid solar thermal panel of claim 17, wherein the support base includes integral inlet and outlet air passages for the rigid solar thermal panel.
 19. The rigid solar thermal panel of claim 17, wherein the support base includes side walls and channels that increase structural rigidity of the solar thermal panel.
 20. The rigid solar thermal panel of claim 17, further comprising a support stand comprising a hinge mechanism adjustable to vary a vertical angle of the solar thermal panel relative to the support stand.
 21. The rigid solar thermal panel of claim 20, wherein the hinge mechanism comprises a substantially cylindrical component, a brace rotatable about the substantially cylindrical component, a series of radial holes in the brace, and a locking pin, the locking pin preventing rotational movement of the brace about the substantially cylindrical component when the locking pin is located in a radial hole.
 22. A method of manufacturing a rigid solar thermal panel, comprising: forming dimples in a metal absorber sheet and side walls along at least two edges of the metal absorber sheet to form a solar absorber layer; placing an optical film on a surface of the solar absorber layer; applying a force substantially perpendicular to the surface of the solar absorber layer to cause the metal absorber layer to bow; securing the optical film on either side of the solar absorber layer proximate to the side walls; and releasing the force to permit the solar absorber layer to return to its pre-bowed state and to tension the optical film across the surface of the metal absorber layer.
 23. The method of claim 22, further comprising placing a spring form at the bonded edges of the optical film.
 24. The method of claim 22, further comprising applying wear strips on at least two edges of the optical film, the wear strips preventing direct contact between the film the spring form. 